Large aluminum nitride crystals with reduced defects and methods of making them

ABSTRACT

Reducing the microvoid (MV) density in AlN ameliorates numerous problems related to cracking during crystal growth, etch pit generation during the polishing, reduction of the optical transparency in an AlN wafer, and, possibly, growth pit formation during epitaxial growth of AlN and/or AlGaN. This facilitates practical crystal production strategies and the formation of large, bulk AlN crystals with low defect densities—e.g., a dislocation density below 10 4  cm −2  and an inclusion density below 10 4  cm −3  and/or a MV density below 10 4  cm −3 .

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefits of and priority to U.S.Provisional Application Ser. No. 60/740,082, filed on Nov. 28, 2005, theentire disclosure of which is hereby incorporated by reference.

GOVERNMENT SUPPORT

This invention was made with United States Government support under70NANB4H3051 awarded by the National Institute of Standards andTechnology (NIST). The United States Government has certain rights inthe invention.

BACKGROUND

Aluminum nitride (AlN) holds great promise as a semiconductor basismaterial for numerous applications, e.g., opto-electronic devices suchas short-wavelength LEDs and lasers, dielectric layers in opticalstorage media, electronic substrates, and chip carriers where highthermal conductivity is essential, among many others. In principle, theproperties of AlN will allow light emission in the 200 nm wavelengthregion to be achieved. But many practical difficulties should beaddressed for such devices to become commercially practicable.

For example, bulk AlN crystals often exhibit a substantial amount ofcracking, which results in crystal separation before, during or afterthe crystal is formed into a wafer. If the crystal cracks or fullyseparates, it is very difficult or sometimes impossible to use it as areliable substrate for device fabrication. Most of the commerciallyavailable machines for epitaxy, photolithography and other deviceprocessing require perfectly shaped, round wafers with uniformthickness. Any crack, even ones that do not result in wafer separation,will impair commercial usefulness. Therefore, the cracking problem inAlN crystal growth has crucial importance for the further development ofnitride-based electronics.

In addition, many opto-electronic applications will require transparentwafers. While AlN is intrinsically transparent at optical wavelengthsbetween 210 and 4500 nm, macroscopic defects such as cracks andinclusions significantly scatter light and reduce the apparenttransparency in this important optical region. Elimination of cracks andinclusions is of critical importance to the development of ultravioletlight emitting diodes (LEDs), for example.

DESCRIPTION OF THE INVENTION Brief Summary of the Invention

Microvoids (MVs) in AlN single crystals are a type of crystallographicdefect that can be classified as an inclusion. The lateral (i.e.parallel to the c-plane) size of the MV defects typically varies from0.1 to 3 micrometers (μm). Under low resolution (≦100×) opticalmicroscopy, the MVs appear to be spherically shaped inclusions. However,under higher (≧200×) resolution optical microscopy, as well as whenimaged using Atomic Force Microscopy (AFM), the MVs appear to have ahexagonal shape well aligned with (i.e., approximately parallel to) thec-plane. During chemical treatment (chemical etching orchemical-mechanical polishing), MVs intersecting an AlN surface leaveshallow pits with a round or hexagonal shape. Also, AlN material with asignificant density of MVs tends to etch faster (during chemicaletching) than the material with somewhat lower MV density. Therefore,the MVs can behave as heterogeneous inclusions in the AlN solid phase.The MVs may be associated with a gas-phase segregation, e.g. of oxygen,nitrogen, or hydrogen. MVs containing segregated gaseous species canlower the stacking symmetry of the AlN parallel to the c-axis thusrepresenting stacking faults. In crystals grown bysublimation-recondensation (such as described in U.S. Pat. No.6,770,135, the entire disclosure of which is hereby incorporated byreference), the volume density can be about 10⁹ cm⁻³ and the surfacedensity of MVs (i.e., MVs which either intersect the surface aftercutting or are within 1 μm of the surface) can be about 10⁵ cm⁻².

We have found that reducing the MV density in AlN ameliorates numerousproblems related to cracking during crystal growth, etch pit generationduring the polishing, reduction of the optical transparency in an AlNsubstrate, and, possibly, growth pit formation during epitaxial growthof AlN and/or AlGaN. The ability to address these problems by reducingMV formation facilitates practical crystal production strategies and theformation of large, bulk AlN crystals with low defect densities—e.g., adislocation density below 10⁴ cm⁻², an inclusion density below 10⁴ cm⁻³(MVs are a type of inclusion) and/or a MV density below 10⁴ cm⁻³. Theinvention also facilitates fabrication of single-crystal wafers of AlNready for epitaxial growth and oriented within 2° of the (0001) face(c-face) with the Al polarity, and having a shallow pit density of lessthan 100 cm⁻². Embodiments of the invention further facilitate formationof single-crystal wafers of AlN that are at least 2 cm in diameterhaving an optical absorption coefficient of less than 1 cm⁻¹ over anypart of the wavelength range where AlN is intrinsically transparentspanning from 210 nm to 4,500 nm. Embodiments of the invention alsopermit fabrication of substantially crack-free boules of single-crystalAlN having diameters larger than 2 cm and lengths greater than 1 cm. Theterm “wafer,” as utilized herein, refers to a self-supporting substrateand/or portion of a boule of material such as AlN. The term“substantially crack-free,” as utilized herein, refers to a crackdensity of less than 5 cracks per boule or volume of single crystalmaterial which is approximately 2 cm diameter by 1 cm in length, or lessthan 10 cracks per two-inch (2″) diameter wafer, where the cracks aretypically less than approximately 2 cm in length. Alternatively, it canrefer to approximately zero cracks in a crystal boule or portionthereof, or can refer to a finite crack density insufficient to causeseparation of a portion of the crystal boule during growth, cool-down,slicing, or other handling procedures.

Accordingly, in a first aspect the invention features a method ofgrowing single-crystal AlN including providing in a crystal growthenclosure a vapor comprising Al and N₂, and depositing the vapor assingle-crystalline AlN having a microvoid density less thanapproximately 10⁴ cm⁻³. In an embodiment, the partial pressure of N₂ inthe crystal growth enclosure may be maintained at a level greater than astoichiometric pressure relative to the Al. The partial pressure of N₂may be within the range of 1-50 bar.

One or more of the following features may be included. A growth rate ofthe single-crystalline AlN in any crystallographic direction may bewithin the range of approximately 0.1 to approximately 2 mm/hr. A pushrate less than or equal to the intrinsic growth rate of thesingle-crystalline AlN may be maintained. The push rate may be at least0.1 mm/hr. Deposition of the single-crystalline AlN may originate at aseed crystal disposed within the crystal growth enclosure and orientedsuch that a direction of maximum growth rate of the single-crystallineAlN is rotated at least approximately 10° away from a c-axis of thesingle-crystalline AlN and toward a non-polar direction. The oxygenconcentration in the vapor may be less than 300 parts per million (ppm)atomic percent, and the hydrogen concentration in the vapor may be lessthan 0.5%. The crystal growth enclosure may include tungsten, and thegrowth temperature may be less than approximately 2350° C. The crystalgrowth enclosure may include at least one of tantalum and carbon, andthe growth temperature may be less than approximately 2750° C. Thetemperature gradient along a length of the crystal growth enclosure maybe greater than approximately 5° C./cm and less than approximately 100°C./cm.

The method may further include, following deposition, the step ofslicing an AlN wafer from the single-crystalline AlN. Thecross-sectional area of the AlN wafer may be approximately equal to thatof the single-crystalline AlN, and may be approximately circular with adiameter greater than approximately 2 cm.

One or more of the following features may be included. The method mayinclude the step of annealing the AlN wafer at a first temperature and afirst pressure. The first temperature may be greater than approximately2000° C. and the first pressure may be greater than approximately 35bar. The first temperature may be less than approximately 2350° C. Afterannealing, the microvoid density in a center region of the AlN wafer maybe less than the density in an edge region of the AlN wafer, and may beless than approximately 10⁴ cm⁻³, or even approximately zero.

The method may include the step of polishing the AlN wafer, whereinafter polishing the AlN wafer has an etch pit density less thanapproximately 100 cm⁻². In some embodiments, the method includes thestep of depositing an epitaxial layer on the AlN wafer, wherein theepitaxial layer has a surface pit density less than approximately 100cm⁻². In another embodiment, the microvoid density of thesingle-crystalline AlN is greater than zero and the single-crystallineAlN is substantially crack-free.

In another aspect, the invention features a bulk AlN crystal having adislocation density below 10⁴ cm⁻² and an inclusion density below 10⁴cm⁻³. In an embodiment, the microvoid density of the crystal is lessthan approximately 10⁴ cm⁻³.

In yet another aspect, the invention features a single-crystal wafer ofAlN oriented within 2° of the (0001) face with the Al polarity, thewafer having a shallow pit density less than 100 cm⁻². In an embodiment,the wafer is substantially crack-free. The microvoid density in a centerregion of the wafer may be less than the density in an edge region ofthe wafer. For example, the microvoid density in a center region of thewafer may be approximately zero and the microvoid density in an edgeregion of the wafer may be less than approximately 10⁴ cm⁻³.

In another aspect, the invention features a single-crystal wafer of AlNhaving an optical absorption coefficient of less than 5 cm⁻¹ at allwavelengths in the range spanning 500 nm to 3,000 nm. In an embodiment,the wafer is substantially crack-free. A microvoid density in a centerregion of the wafer may be less than a microvoid density in an edgeregion of the wafer. The microvoid density in a center region of thewafer may be approximately zero and the microvoid density in an edgeregion of the wafer may be less than approximately 10⁴ cm⁻³. In anembodiment, the optical absorption coefficient may be less than 1 cm⁻¹at all wavelengths in the range spanning 500 nm to 3,000 nm. The wafermay have a diameter greater than approximately 2 cm.

In another aspect, the invention features a single-crystal wafer of AlNhaving an optical absorption coefficient less than 1 cm⁻¹ at anywavelength in the range spanning 210 nm to 4,500 nm and a diametergreater than approximately 2 cm.

In yet another aspect, the invention features a boule of crack-free,single-crystal AlN having a diameter larger than 2 cm and a lengthgreater than 1 cm. In an embodiment, the boule may have a microvoiddensity in the range of greater than zero to less than approximately 10⁴cm⁻³.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 schematically depicts a crystal growth enclosure for the growthof single-crystalline AlN;

FIGS. 2A and 2B schematically depict an AlN wafer separated from a bouleof single-crystalline AlN;

FIG. 3 schematically depicts an AlN wafer exhibiting domains ofdiffering MV density; and

FIG. 4 schematically depicts an epitaxial layer deposited on an AlNwafer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Coalescence of MVs can result in their lining up parallel to theprismatic {1 100} planes (i.e., m-planes) in an AlN crystal. In severalcases we observed MVs creating tree-like features where the treebranches represent lining up of MVs that often evolve into a solid tree(crack or micro-crack). Another type of crack formation due to MVcoalescence—a rare but still significant case—occurs when MVs form a fewcircles from their common center from which the crack spreads out almostto 180°. Moreover, since some of the MVs intersect the surface, they caninfluence the polishing process by producing etch pits.

MVs can reduce the optical transparency of an AlN substrate. To firstorder, the reduction in transparency can be modeled by assuming that theamount of radiation scattered by the MVs is proportional to theeffective area presented by the MVs. At low densities of MVs, this meansthat the reduction in transmission of radiation through a slab of AlNwith thickness d, will be equal to ndA where n is the numerical densityof MVs and A is the effective area each MV presents to scatter theradiation. The transmission T through a uniform slab of material ofthickness d is normally expressed in terms of an absorption coefficientα such that T=e^(−αd). In this model, the absorption coefficient will beapproximately equal to nA and will be independent of wavelength. If theeffective area A that the MVs present to scatter radiation is ˜1 μm²,then the density of MVs is preferably below ˜10⁸ cm⁻³ in order toachieve an absorption coefficient that is less than 1 cm⁻¹. Indeed, thiscalculation corresponds to optical absorption measurements on AlN.

The three distinct colorations of AlN substrate wafers are observed.Depending on growth conditions, sometimes these colorations can all beobserved in a single-crystal AlN wafer. The colorations can be describedas:

1) Dark (brownish);

2) Yellow (amber); and

3) Light yellow.

The dark area usually contains a high density of MVs that are easilyobserved. MVs are harder to observe in the yellow area due to the lackof optical contrast (MVs and amber material are both bright). Also, insome cases, the density of MVs in the yellow area is smaller than thatin darker region. It is even harder to see MVs in large yellow regions,but again, that may be due to the contrast or because light yellowregions represent grains that are usually oriented strongly off-axis. Itappears, however, that the most important difference between the darkand yellow regions is that MVs in the dark region are normally connectedto each other, creating a network, while the MVs in the yellow areas donot exhibit this feature.

The present invention also stems from the recognition that generation ofMVs during AlN crystal growth makes it easier for the AlN crystal tocrack. Therefore, embodiments of the present invention prevent or reduceMV generation in order to prevent or reduce cracking. This may beaccomplished through adjustment and control of one or more of:

1) The expansion rate of the crystal;

2) The gas mixture;

3) The ambient pressure;

4) The growth temperature;

5) The temperature gradients;

6) The push rate; and/or

7) The post-growth annealing.

1) Expansion Rate

There are two main directions of the crystal expansion during crystalgrowth: lateral and vertical. MV formation can be controlled bycontrolling the expansion rate because MVs can form due to vacancyaggregation. If the AlN crystal is grown under stochiometric gas phaseconditions (i.e., the number of Al and N atoms in the vapor phase isequal), then there will not be enough single nitrogen (N) atoms to bondto each aluminum atom in the crystal lattice due to the very highbonding energy of the N₂ molecule (˜9.76 eV at 0 K). The overall effectof single N atoms provided to the growth surface can be described byintroducing an effective accommodation coefficient that is less than 1if the number of single N atoms at the growing surface is less than thenumber of N₂ molecules arriving at the surface. When the number ofsingle N atoms incorporated into the crystal is less than the number ofthe Al atoms, excess Al atoms will be trapped in the crystal.

Too high a growth rate (expansion) in any direction will result in thetrapping of excess Al atoms in the crystal, thus creating N vacancies.These N vacancies, in turn, can later aggregate and generate MVs. Inorder to minimize or completely avoid formation of MVs, the growth ratein any crystallographic direction is preferably kept in the range from0.1 to 2 mm/hr. The lower growth rate limit is defined by practicalconsiderations for creating bulk crystals from vapor-phase constituents,while the upper growth limit is defined by the desired crystallinequality of grown crystal: as the growth rate increases, the fracture ofpolycrystalline material increases as well. The maximum growth rate willalso depend on the crystallographic orientation of the growing crystal.

It is important to note that a expansion rate may result in generatingdifferent types of point defects (vacancies) such as pairs of Al and Nvacancies (see, e.g., G. A. Slack, L. J. Schowalter, D. Morelli, J. A.Freitas Jr., J. Crystal Growth, vol. 246, p. 287 (2002), the disclosureof which is hereby incorporated by reference), especially when oxygen ispresent in the vapor phase. Any vacancies can aggregate and generateMVs.

Hence, referring to FIG. 1, according to various embodiments, duringgrowth of single-crystalline AlN, a crystal growth enclosure 100contains a vapor mixture 110 and a single-crystal AlN boule 120. Vapormixture 110 includes primarily gaseous Al and N₂, and arises from thesublimation of source 160. Boule 120 is formed by the recondensation ofvapor mixture 110 at a top end of crystal growth enclosure 100, and mayoriginate from a seed crystal (not pictured). The growth rate of boule120 can be defined as the increase in size of boule 120, for example,parallel or perpendicular to longitudinal axis 150 of crystal growthenclosure 100. The direction of maximum growth rate of boule 120 isgenerally along longitudinal axis 150. The MV density of boule 120 canbe kept below a level of approximately 10⁴ cm⁻³ by maintaining thegrowth rate of boule 120 in the range of 0.1 to 2 mm/hr. In anembodiment, the MV density of boule 120 is greater than zero. The seedcrystal may be oriented such that the direction of maximum growth rateof boule 120 is oriented at least approximately 10° away from the c-axisand towards a non-polar direction, i.e., a c-axis of boule 120 may beoriented at least approximately 10° away from longitudinal axis 150.Such a seed crystal orientation may act to suppress MV formation.

2) Gas Mixture

The purity and the composition of the gas mixture are importantparameters in controlling generation of MVs. Forming gas containshydrogen (usually ≦5% by volume) and is used in order to reduce possibleoxygen contamination. However, hydrogen molecules can be easily trappedin the AlN material (as is the case with other semiconductors as well).By diffusion as interstitials, these hydrogen atoms can agglomerate andcreate MVs. Oxygen contamination may result from several sourcesincluding residual water vapor and contaminated supply gases. Once inthe crystal crucible, it may be transported as Al₂O in the vapor phaseand attach to the Al sites in the crystal lattice, thus creating Alvacancies. The Al vacancies can aggregate and create MVs. Accordingly,using high-purity gases with low amounts of hydrogen and oxygen willreduce the probability of MV formation. The use of UHP-grade N₂ gas,applicable in the semiconductor industry, is desirable. However,additional steps will need to be taken, including the use of nitrogengas filters to purify the N₂ gas being introduced, as well as the use ofgas flow to remove contamination from the furnace elements as they heatup. For example, the UHP-grade nitrogen gas can be passed through anAeronex filter (model SS2500KFH4R, max flow 300 SLM, filtration 0.003μm). The delivery rate to the hot zone can be 0.125 liter per minute(LPM) although the optimum gas volume delivered to the reaction zonewill vary based on the specific geometry.

Thus, in an embodiment, the concentration of hydrogen in vapor mixture110 is less than 0.5%. In another embodiment, the concentration ofoxygen in vapor mixture 110 is less than 300 parts per million (ppm)atomic percent. Maintenance of low hydrogen and oxygen concentrations invapor mixture 110 prevent formation of point defects in boule 120 thatcan agglomerate and form MVs. Thus, the MV density in boule 120 can bemaintained below approximately 10⁴ cm⁻³. In an embodiment, the MVdensity of boule 120 is greater than zero.

3) Ambient Pressure

The evaporation of AlN is nearly congruent. The equilibrium vapor phaseof evaporated AlN consists almost exclusively of Al atoms and N₂molecules at the temperatures typically used for crystal growth (1900 to2450° C.). For the AlN crystal to grow, N₂ molecules that are adsorbedon the surface must be broken into atomic N in order to be incorporatedinto the growing crystal. As stated above, the lack of sufficient Natoms present at the growth surface results in generation of N vacanciesthat can cause formation of MVs. In order to provide a sufficient sourceof N atoms, the partial pressure of N₂ is preferably increased wellabove the stochiometric value (or, alternatively, the growth temperatureincreased). The higher the ambient N₂ pressure, the higher the amount ofatomic N that will be generated on the growing crystal surface andavailable for incorporation into the crystal. Thus, higher N₂ partialpressure reduces the probability of MV formation. The ambient N₂pressure is preferably, therefore, kept as high as possible in order toprovide sufficient atomic N flux to the growing interface; see, e.g.,U.S. Pat. No. 6,770,135. The ranges needed to grow AlN crystals fromvapor phase are 1-50 bar where the lower limit is defined by the lowestN₂ pressure required to obtain high quality AlN crystals, while theupper limit is defined by the reasonable growth rate as increasing theambient pressure results in a diffusion-limited growth regime.

Thus, in an embodiment, vapor mixture 110 contains a partial pressure ofN₂ greater than the stoichiometric pressure relative to the partialpressure of Al, i.e., the number of N atoms in vapor mixture 110 isgreater than the number of Al atoms in vapor mixture 110. Maintenance ofthis high partial pressure of N₂ relative to Al prevents the formationof N vacancies which can agglomerate and form MVs. In an embodiment, thepartial pressure of N₂ in vapor mixture 110 is in the range of 1 to 50bar. Thus, the MV density in boule 120 can be maintained belowapproximately 10⁴ cm⁻³. In an embodiment, the MV density of boule 120 isgreater than zero.

4) Growth Temperature and 5) Temperature Gradients

An insufficient growth temperature results in polycrystalline growth,which increases the defect density and, therefore, the formation of MVs.Moreover, lower growth temperatures decrease the surface concentrationsof monatomic N and therefore lead to N vacancies, which, in turn,further contribute to MV formation. Very high temperature gradients alsoresult in polycrystalline growth and should also be avoided in order toreduce the possibility of MV generation. The highest acceptable growthtemperature is generally limited by the possible formation of eutecticsolutions of AlN with the crucible, so the temperature regimes employedshould be judiciously chosen. For instance, the highest possible growthtemperature at 1 bar pressure would be 2330° C. (see, e.g., Glen A.Slack and T. F. McNelly, “Growth of high purity AlN crystals,” J Cryst.Growth, vol. 34, pp. 263-279 (1976) and Glen A. Slack, Jon Whitlock, KenMorgan, and Leo J. Schowalter, “Properties of Crucible Materials forBulk Growth of AlN,” Mat. Res. Soc. Symp. Proc. Vol. 798, p. Y10.74.1(2004) (“Slack 2004”), the entire disclosures of which are herebyincorporated by reference), since above this temperature AlN will form aliquid eutectic with the crucible and destroy it. Higher temperaturescan be achieved by using other crucible materials such as suggested inSlack 2004. However, problems with contamination and leakage through thecrucible walls may be anticipated. Leakage through polycrystallinecrucibles may be addressed using the techniques described in U.S. Pat.No. 6,719,843 and in U.S. patent application Ser. No. 10/822,336, theentire disclosures of which are hereby incorporated by reference.

In any case, the maximum temperature of growth bysublimation-recondensation will be limited to less than approximately2750° C. Above, this temperature, AlN has been observed to melt at anelevated nitrogen pressure of 10 megaPascals (MPa) (see V. L.Vinogradov, A. V. Kostanovskii, and A. V. Kirillin, “Determination ofthe Melting Parameters of Aluminium Nitride,” High Temperatures—HighPressures, vol. 23, p. 685 (1991), the disclosure of which is hereinincorporated by reference). It is anticipated that the highesttemperature gradient at which MV formation will be diminished is lessthan approximately 100° C./cm. The temperature gradient is related toboth expansion rate and vertical growth rate. In case of MV formationdue to too high an expansion rate or vertical growth rate, thencontrolling the temperature gradients (radial and axial) can limit theMV formation as well as their further migration and agglomeration.

Referring to FIG. 1, heat source 140 surrounds crystal growth enclosure100 and regulates the growth temperature therein. Moreover, the localtemperature at various points in heat source 140, as well as the speedof travel of crystal growth enclosure 100 through heat source 140 (i.e.,the push rate described below), control the thermal gradient in boule120. The thermal gradient is defined herein as the change in temperatureof the boule as a function of distance along the length L of the crystalgrowth enclosure.

In an embodiment, heat source 140 is regulated such that the growthtemperature inside crystal growth enclosure 100 is less thanapproximately 2350° C. and crystal growth enclosure 100 is madeprimarily of tungsten. In a preferred embodiment, crystal growthenclosure 100 is made primarily of tungsten and the growth temperaturefalls within a range of 1900 to 2350° C. In another embodiment, crystalgrowth enclosure 100 is made primarily of niobium carbide (NbC) and thegrowth temperature falls within a range of 1900 to 2350° C. In anotherembodiment, crystal growth enclosure 100 is made primarily of tantalumcarbide (TaC) or tantalum (Ta) coated with a layer of carbon (C) and thegrowth temperature falls within a range of 1900 to 2400° C. The layer ofC may be formed on crystal growth enclosure 100 made primarily of Ta bychemical vapor deposition. In another embodiment, crystal growthenclosure 100 is made primarily of C and the growth temperature fallswithin a range of 1900 to 2750° C., or, preferably, within a range of1900 to 2550° C.

In an embodiment, the thermal gradient of boule 120 is maintained at alevel greater than approximately 5° C./cm and less than approximately100° C./cm. This high thermal gradient prevents the formation of MVs ata level greater than approximately 10⁴ cm⁻³, and maintains growth ofboule 120 as a single crystal rather than as polycrystalline material.In an embodiment, the MV density of boule 120 is greater than zero.

6) Push Rate

There is an optimum push rate at which MV generation is negligible.Preferably, the push rate is slightly less than or equal to theintrinsic, or maximum, growth rate (as stated above, 0.1-2 mm/hr). Ifthis condition is not obeyed, then the crystal growth rate in the growthdirection may be too high, resulting in MV formation as explained above.Moreover, a high push rate may lead to the nucleation of AlN on thewalls of a tungsten crucible, which, in turn, will increase the defectdensity and the possibility of MV generation. In addition, higher pushrates may result in the predominate growth of other planes rather thanc-planes, an effect that further contributes to MV formation.

However, the push rate should not be too low (<0.1 mm/hr) since atungsten crucible degrades if exposed to the Al vapor for a very longtime. The Al vapor attacks the crucible wall along its grain boundariesand other defects, resulting in leakage of some Al vapor and consequentpore formation in the growing material. Therefore, it is desirable tochoose the push rate so as to reduce MV formation.

Referring to FIG. 1, crystal growth enclosure 100 also includes pushmechanism 130, which controls the travel of crystal growth enclosure 100through surrounding heat source 140, thereby controlling both the pushrate and the thermal gradient along the longitudinal axis 150. The pushrate is defined as the speed at which push mechanism 130 propels crystalgrowth enclosure 100 through heat source 140. In an embodiment, the pushrate is maintained at a level less than or approximately equal to amaximum growth rate of boule 120 in the direction parallel to thelongitudinal axis 150. Here, the maximum growth rate is defined by themaximum rate allowed by the crystal growth temperature and the sourcetemperature. In a preferred embodiment, the push rate is maintained at alevel of approximately one-half the maximum growth rate of boule 120.Thus, the MV density in boule 120 can be maintained below approximately10⁴ cm⁻³. In an embodiment, the MV density of boule 120 is greater thanzero.

As used herein, the push rate can also be defined as an actual growthrate maintained at a level below that enabled by the specific growthconditions, e.g., the growth temperature and the source temperature.Maintenance of a specific push rate can also be equivalentlyaccomplished without physical movement of crystal growth enclosure 100.For example, heat source 140 could be moved relative to a stationarycrystal growth enclosure 100, or the distance or temperature gradientbetween boule 120 and source 160 can be altered. Generally, any of thesemethods in which actual growth rate is controlled at a level below themaximum intrinsic growth rate can be utilized interchangeably with pushrate.

7) Post-Growth Annealing

Referring to FIGS. 2A and 2B, wafer 200 may be sliced from boule 120 bythe use of, for example, a diamond annular saw or a wire saw.Maintaining an MV density less than approximately 10⁴ cm⁻³ in boule 120substantially prevents crack formation in both boule 120 and wafer 200.Such cracks can result in, among other effects, separation of smallportions of either boule 120 or wafer 200 during wafer slicing. Thus,through the prevention of crack formation, a surface 210 of wafer 200may have substantially the same shape and cross-sectional area of asurface 220 of boule 120. In an embodiment, surface 210 may beapproximately circular in shape and have a diameter of greater thanapproximately 2 cm. In an alternate embodiment, surface 210 may be aquadrilateral or other polygon with an area greater than approximately 3cm².

In an embodiment, a crystalline orientation of surface 210 may be withinapproximately 2° of the (0001) face (i.e., the c-face) and have an Alpolarity. In other embodiments, surface 210 may have a N polarity or beoriented within approximately 2° of the m-face or a-face orientation.

Referring to FIG. 3, wafer 200 may be annealed to reduce the density ofMVs therein. The annealing temperature may be in the range spanning 1000to 2350° C. In a preferred embodiment, the annealing temperature isapproximately 2000° C. In yet another embodiment, wafer 200 is subjectedto high pressure during annealing. The annealing pressure may beselected from the range of 1 to 50 bar, and in a preferred embodiment,the annealing pressure is approximately 35 bar. In an embodiment, wafer200 may be annealed in an ambient including N₂ gas. In anotherembodiment, wafer 200 may be annealed at a temperature of approximately1200° C. and a pressure of approximately 2 bar.

Annealing of wafer 200 may reduce the MV density thereof from a level ofapproximately 10³ to 10⁹ cm⁻³ to a level of approximately zero to 10³cm⁻³. Hence, the MV density of wafer 200 after annealing will be lessthan that of boule 120 from which it was sliced.

Reducing the MV density of wafer 200 may also improve its opticaltransparency. In an embodiment, an optical absorption coefficient ofwafer 200 is less than 1 cm⁻¹ at any wavelength within the range ofapproximately 210 nm to approximately 4,500 nm. In another embodiment,the optical absorption coefficient of wafer 200 is less than 5 cm⁻¹ atall wavelengths within the range of approximately 500 nm toapproximately 3,000 nm. The optical absorption coefficient may be lessthan 1 cm⁻¹. Wafer 200 may have a diameter larger than approximately 2cm.

Annealing wafer 200 may also result in a non-uniform distribution of MVsacross wafer 200. Referring to FIG. 3, after annealing, a center region300 may have a lower MV density than an edge region 320. For example,the MV density in center region 300 may be in the range of approximatelyzero to approximately 10² cm⁻³, and the MV density in edge region 320may be in the range of approximately 10³ cm⁻³ to approximately 10⁴ cm⁻³.In an embodiment, boundary area 310 separates center region 300 fromedge region 320. In an embodiment, center region 300 is approximatelycircular in shape and has a diameter of 1-3 mm, and edge region 320conforms to an outer boundary of wafer 200 and has a width ofapproximately 3 mm to approximately 50 mm. Generally, the size of eithercenter region 300 or edge region 320 may be larger than described above,with the other region correspondingly shrinking in size. The utilizationof longer anneal times and/or higher annealing temperatures may resultin the growth of center region 300 at the expense of edge region 320,i.e., boundary region 310 may effectively move toward the outer edge ofwafer 200 as MVs are eliminated from wafer 200. In an embodiment,annealing proceeds for a time sufficient for boundary region 310 toreach the outer edge of wafer 200, i.e., substantially all of the areaof wafer 200 will have a reduced density of MVs. In an embodiment, anoverall MV density of wafer 200 may be within the range of approximatelyzero to approximately 10² cm⁻³.

Referring to FIG. 4, epitaxial layer 400 may be deposited on a surfaceof wafer 200 for the purposes of subsequent device fabrication.Epitaxial layer 400 may include AlN, GaN, InN, and/or binary andtertiary alloys of each. Prior to epitaxial deposition, wafer 200 may bepolished in order to improve the quality and planarity of a surfacethereof. Various polishing techniques may be utilized, e.g., the methodsdescribed in U.S. patent application Ser. No. 11/448,595 and U.S. Pat.No. 7,037,838, the entire disclosures of which are hereby incorporatedby reference. In an embodiment, wafer 200 is polished ofchemical-mechanical polishing (CMP). Control of MV formation in wafer200 may result in a superior post-polish surface of wafer 200, as fewMVs will intersect the surface of wafer 200 and be etched preferentiallyduring polishing. In an embodiment, after polishing, wafer 200 will havean etch pit density less than approximately 100 cm⁻². Control of MVformation during crystal growth and/or etch pit formation duringpolishing may facilitate the deposition of epitaxial layer 400 with ashallow pit density less than approximately 100 cm⁻². Wafer 200, with orwithout epitaxial layer 400, desirably is substantially crack-free.

It will be seen that the techniques described herein provide a basis forimproved production of AlN crystals. The terms and expressions employedherein are used as terms of description and not of limitation, and thereis no intention in the use of such terms and expressions of excludingany equivalents of the features shown and described or portions thereof.Instead, it is recognized that various modifications are possible withinthe scope of the invention claimed.

1. A method of growing single-crystal AlN, the method comprising the steps of: a. providing in a crystal growth enclosure a vapor comprising Al and N₂; and b. depositing the vapor as single-crystalline AlN having a microvoid density less than approximately 10⁴ cm⁻³ while pushing the crystal growth enclosure at a rate less than an intrinsic growth rate of the AlN.
 2. The method of claim 1, further comprising the step of maintaining, in the crystal growth enclosure, a partial pressure of N₂ greater than a stoichiometric pressure relative to the Al.
 3. The method of claim 2, wherein the partial pressure of N₂ is within a range of 1-50 bar.
 4. The method of claim 1, wherein the growth rate of the single-crystalline AlN in any crystallographic direction is within a range of approximately 0.1 to approximately 2 mm/hr.
 5. The method of claim 1, wherein the push rate is at least 0.1 mm/hr.
 6. The method of claim 1, wherein a concentration of oxygen in the vapor is less than 300 ppm and a concentration of hydrogen is less than 0.5%.
 7. The method of claim 1, wherein the crystal growth enclosure comprises tungsten and a growth temperature is less than approximately 2350° C.
 8. The method of claim 1, wherein the crystal growth enclosure comprises at least one of tantalum and carbon, and a growth temperature is less than approximately 2750° C.
 9. The method of claim 1, further comprising maintaining a temperature gradient along a length of the crystal growth enclosure of greater than approximately 5° C./cm and less than approximately 100° C./cm.
 10. The method of claim 1, further comprising, following deposition, the step of slicing a AlN wafer from the single-crystalline AlN.
 11. The method of claim 10, wherein a cross-sectional area of the AlN wafer is approximately equal to a cross-sectional area of the single-crystalline AlN.
 12. The method of claim 11, wherein the cross-sectional area of the AlN wafer is approximately circular with a diameter of greater than approximately 2 cm.
 13. The method of claim 10, further comprising the step of annealing the AlN wafer at a first temperature and a first pressure.
 14. The method of claim 13, further comprising the step of depositing an epitaxial layer on the AlN wafer after the AlN wafer is annealed.
 15. The method of claim 1, further comprising: following deposition, slicing a AlN wafer from the single-crystalline AlN; and polishing the AlN wafer, wherein after polishing the AlN wafer has an etch pit density less than approximately 100 cm⁻².
 16. The method of claim 15, further comprising the step of depositing an epitaxial layer on the AlN wafer, wherein the epitaxial layer has a surface pit density less than approximately 100 cm⁻².
 17. The method of claim 1, wherein the microvoid density is greater than zero and the single-crystalline AlN is substantially crack-free.
 18. A method of growing single-crystal AlN, the method comprising the steps of: a. providing in a crystal growth enclosure a vapor comprising Al and N₂; and b. depositing the vapor as single-crystalline AlN having a microvoid density less than approximately 10⁴ cm⁻³, wherein the deposition of the single-crystalline AlN originates at a seed crystal disposed within the crystal growth enclosure and oriented such that a direction of maximum growth rate of the single-crystalline AlN is rotated at least approximately 10° away from a c-axis of the single-crystalline AlN and toward a non-polar direction.
 19. A method of growing single-crystal AlN, the method comprising the steps of: a. providing in a crystal growth enclosure a vapor comprising Al and N₂; b. depositing the vapor as single-crystalline AlN having a microvoid density less than approximately 10⁴ cm⁻³; c. following deposition, slicing a AlN wafer from the single-crystalline AlN; and d. annealing the AlN wafer at a first temperature and a first pressure, wherein the first temperature is greater than approximately 2000° C. and the first pressure is greater than approximately 35 bar.
 20. The method of claim 19, wherein the first temperature is less than approximately 2350° C.
 21. A method of growing single-crystal AlN, the method comprising the steps of: a. providing in a crystal growth enclosure a vapor comprising Al and N₂; b. depositing the vapor as single-crystalline AlN having a microvoid density less than approximately 10⁴ cm⁻³; c. following deposition, slicing a AlN wafer from the single-crystalline AlN; and d. annealing the AlN wafer at a first temperature and a first pressure, wherein after annealing, a microvoid density in a center region of the AlN wafer is less than a microvoid density in an edge region of the AlN wafer.
 22. The method of claim 21, wherein the microvoid density in a center region of the AlN wafer is less than approximately 10⁴ cm⁻³.
 23. The method of claim 22, wherein the microvoid density in a center region of the AlN wafer is approximately zero. 